The present invention relates in general to the transfer of energy by means of electromagnetic waves to power a remote device. More specifically, the present invention relates to a system for "power beaming" energy from a source at high frequencies and rectifying such energy to provide a source of DC energy to a remote device.
Attempts have been made for many years to develop a system for beaming energy from a source to power a remote device with a high degree of efficiency (for a general discussion see "The History of Power Transmission by Radio Waves" by William C. Brown, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-32, No. 9, September 1984). In particular, the concept of powering a satellite or free flying aircraft by power beaming has received a great deal of attention. The advantages of such a system are readily apparent, for example, an aircraft could be maintained on station indefinitely to act as a low cost communications or reconnaissance platform. Early concepts included the conversion of microwave energy into thermal energy to power a helicopter type platform as illustrated in U.S. Pat. 4,542,316 issued to Hart. A more practical approach, however, has focused on converting the microwave energy into DC energy to directly power the platform.
The practical conversion of microwave energy to DC energy for power beaming purposes has been based on the use of rectennas to receive and rectify the microwave energy. Generally, rectennas are limited in their power-handling capabilities, but can be a highly efficient means of converting microwave energy into DC energy for power beaming purposes when employed in large numbers in an array structure. U.S. Pat. 3,434,678 issued to Brown et al. illustrates the use of a rectenna array to power a helicopter platform by power beaming.
More recently, a scale model of a long endurance high altitude platform powered by microwave energy known as SHARP (Stationary High Altitude Relay Platform) has been successfully demonstrated. See "A Microwave Powered High Altitude Platform" by Schlesak et al., 1988 IEEE MTT-S Digest, pp. 283-286. The SHARP concept calls for an array of ground antennas which must be focused on the aircraft. The underside of the aircraft would be coated with a thin-film array of thousands of half-wave dipole rectennas to convert the received microwave energy into DC energy which would be used to power the aircraft's electrical motor. The scale model of the SHARP aircraft was powered by a microwave beam formed from the outputs of two 5 kW continuous-wave magnetrons, which were combined and supplied to a 4.5 meter diameter parabolic antenna to transmit 10 kilowatts of energy at a frequency of 2.45 GHz. Dual polarization rectennas formed of two orthogonal linearly-polarized rectenna arrays were provided on the model aircraft to convert the microwave energy to DC power.
Efforts at power beaming to date, like SHARP discussed above, have focused primarily on using S-band transmission sources due to their ready availability and to reduce power losses due to atmospheric attenuation. S-band power beaming, however, is limited in the amount of power that can be delivered in a practical system. In order to generate sufficient power densities, a large array of ground antennas must be employed which complicates the problem of concentrating the transmitted energy on the aircraft. One could reduce the number of ground antennas employed in the array, but the size of the antennas would increase significantly making them as difficult to track as the array while greatly increasing their expense. In addition, S-band power beaming requires a large amount of surface area for the rectenna array on the aircraft to generate significant power quantities. For example, the SHARP system discussed above would need an array of 100 m.sup.2 of rectenna surface to generate only 35 kW of DC power, 25 kW of which is required to power the propulsion system, while requiring a transmitter having a diameter of 85 meters with an output of 500 kw.